The eye is a specialized sensory organ that is relatively secluded from systemic access by the blood-retinal, blood-aqueous, and blood-vitreous barriers. As a consequence, the eye exhibits some unusual pharmacodynamic and pharmacokinetic properties.
ADME: absorption, distribution, metabolism, excretion
ARMD: age-related macular degeneration
CA: carbonic anhydrase
CAI: carbonic anhydrase inhibitor
CNS: central nervous system
DME: diabetic macular edema
FDA: Food and Drug Administration
GSH: reduced glutathione
HIV: human immunodeficiency virus
IOP: intraocular pressure
IP3: inositol triphosphate
MAO: monoamine oxidase
PLC: phospholipase C
RAR: retinoic acid receptor
RXR: retinoid X receptor
tPA: tissue plasminogen activator
VEGF: vascular endothelial growth factor
The eye is protected by the eyelids and by the orbit, a bony cavity of the skull that has multiple fissures and foramina that conduct nerves, muscles, and vessels (Figure 69–1). In the orbit, connective (i.e., Tenon’s capsule) and adipose tissues and six extraocular muscles support and align the eyes for vision. The retrobulbar region lies immediately behind the eye (or globe). Understanding ocular and orbital anatomy is important for safe periocular drug delivery, including subconjunctival, sub-Tenon, and retrobulbar injections.
Anatomy of the globe in relation to the orbit and eyelids. Routes of administration of anesthesia are represented by the blue needles.
The external surface of the eyelids is covered by a thin layer of skin; the internal surface is lined with the palpebral portion of the conjunctiva, which is a vascularized mucous membrane continuous with the bulbar conjunctiva. At the reflection of the palpebral and bulbar conjunctivae is a space called the fornix, located superiorly and inferiorly behind the upper and lower eyelids, respectively. Topical medications usually are placed in the inferior fornix, also known as the inferior cul-de-sac.
The lacrimal system consists of secretory glandular and excretory ductal elements (Figure 69–2). The secretory system is composed of the main lacrimal gland, which is located in the temporal outer portion of the orbit, and accessory glands located in the conjunctiva. The lacrimal gland is innervated by the autonomic nervous system (Table 69–1 and Chapter 8). The parasympathetic innervation is clinically relevant because a patient may complain of dry eye symptoms while taking medications with anticholinergic side effects, such as tricyclic antidepressants (see Chapter 15), antihistamines (see Chapter 39), and drugs used in the management of Parkinson disease (see Chapter 18). Muscarinic cholinergic and α adrenergic receptors that mediate responses of several pupillary muscles from autonomic nerves also provide means of dilating the pupil for examination of posterior structures.